A current mirror circuit is a circuit which enables transfer from one circuit leg to another of a current supplied, for example, by a reference current source.
Current mirror circuits have many applications in the field of integrated circuits, being particularly useful to bias differential circuits, in the oscillating circuit sector, and in sampling circuits of the sample-and-hold type.
Especially with sample-and-hold circuits, there exists a requirement for currents of large magnitudes, to discharge at a fast rate the capacitances which are typical of such circuits.
It has been proposed to employ, in an IC environment, conventional design current mirrors comprising transistors of the pnp type which occupy, within the integrated circuit itself, a surface area which is at least five times as large as that of a pnp transistor of minimum surface area. This expedient is dictated by the need to transfer high currents, in excess of 1 mA, but involves the serious drawback that a large silicon surface area must be reserved on the integrated circuit, at variance with the trend toward integrating an increasingly higher number of circuits per unit surface area.
A second expedient is that of integrating high capacity current mirrors, of the type comprising an operation amplifier having one input connected to both a reference current source and a power supply pole through a first resistor, and having another input connected to said power supply pole via a second, socalled "feedback", resistor; the amplifier has its output connected to the base of a transistor of the pnp type which supplies a reflected current I.sub.O proportional to the reference current I.sub.R. The proportionality of the current to be transferred to the reference current will depend on the relation between the first resistance in the circuit and the feedback resistance, as well as directly on the .alpha. parameter of the pnp-type transistor which supplies the reflected current.
This second mirror type, while substantially achieving its objectives, has the disadvantage of involving a diminished .alpha. parameter of the pnp transistor and a consequent, not negligible error in the reflected current I.sub.O.